This invention relates to a method of making 3-methoxy2,4,5-trifluorobenzoic acid (MTBA). In particular, it relates to a series of at least four reaction steps for producing MTBA which proceed from a tetrafluorophthalamide to salts of 4-hydroxy-3,5,6-trifluoro-N-alkylphthalamic acid and 3-hydroxy-2,4,5-trifluoro-N-alkylbenzamide to 4-methoxy-3,5,6-trifluoro-N-alkylphthalamic acid and 3-methoxy-2,4,5-trifluoro-N-alkylbenzamide to 4-methoxy-3,5,6-trifluorophthalic acid and MTBA, then to all MTBA.
MTBA is useful as an intermediate in preparing quinolone antibacterials. MTBA is now being made by a process which first involves preparing 4-hydroxy-3,5,6-trifluoro-N-substituted phthalamic acid (HTPA). (See U.S. Pat. No. 5,233,082, herein incorporated by reference.) The HTPA is decarboxylated and methylated to produce MTBA. While that process works very well, for economic reasons it has become desirable to identify other processes for making MTBA.